The present invention relates to a speaker system using a sheet-like diaphragm utilizing a piezoelectric body.
In recent years, a speaker using a sheet-like diaphragm utilizing a piezoelectric body has been actively studied.
For example, JP 4426738 B discloses a speaker in which a piezoelectric ceramic is stuck to a diaphragm such as a resin or a metal, a voltage corresponding to a signal is applied to the piezoelectric ceramic to convert an expansion and contraction motion of the piezoelectric ceramic into a bending motion of the diaphragm, and thereby generating sound (a sound wave).
Furthermore, a speaker utilizing, as a vibration body, a so-called piezoelectric film in which an electrode layer is formed on each of both surfaces of a sheet-like piezoelectric material, such as a polymeric piezoelectric material, for example, a uniaxially stretched polyvinylidene fluoride (PVDF) film, or a polymeric composite piezoelectric body in which piezoelectric ceramic particles in a powder form are dispersed in a polymer material as a matrix, has also been used (JP 2008-294493 A).
In general, in a direct radiator speaker, an output sound pressure from the speaker is proportional to vibration acceleration of a vibration body. Accordingly, in order to make frequency characteristics of output sound constant, it is necessary to make the acceleration of the vibration body constant irrespective of a frequency.
Here, in a vibration model with one degree of freedom such as a cone speaker, in which sound is generated by piston motion of a diaphragm (cone paper) reciprocating back and forth, it is known that a frequency band lower than the lowest resonance frequency is an elasticity control area in which displacement is constant, the vicinity of the resonance frequency is a resistance control area in which speed is constant, and a frequency band higher than the resonance frequency is a mass control area in which acceleration is constant, as conceptually illustrated in FIG. 29. Therefore, in the conventional speaker, the lowest resonance frequency is lowered below a frequency band to be used so as to realize a uniform output sound pressure in the frequency band to be used.
A lowest resonance frequency f0 is expressed as f0=½π×√(s/m) (s: stiffness, and m: mass). Thus, the cone speaker has been devised to have small stiffness and great mass in a vibration system so as to lower the lowest resonance frequency.
Incidentally, since the piezoelectric speaker has a mechanism in which an expansion and contraction motion of a diaphragm (piezoelectric body) itself is converted into a bending motion to generate sound, unlike an ideal piston motion as in the cone speaker, it is necessary for the diaphragm itself to be as soft as possible and be heavy in order to lower the lowest resonance frequency.